Pressurized containment/isolation rooms or other pressurized spaces (such rooms or spaces being hereinafter collectively referred to as rooms) are finding increasing application in industry, research laboratories, medical facilities and other institutions. In particular, negatively pressurized rooms may be utilized to contain contaminants, for example toxic gases, in industrial and laboratory facilities and to isolate infectious patients, for example patients with TB, in medical facilities. Similarly, positively pressurized rooms may be utilized for isolation or to prevent contamination in clean room areas such as those used in the manufacture of semiconductor products and other delicate industrial procedures and to protect immune deficient patients, such as those with AIDS, in a medical facility. Such facilities may have a single pressurized room connected to an external space such as a hall, or may have a number of such rooms connected to a common corridor.
Since, even in well-sealed pressurized rooms, there is some air flow through and around closed doors and through walls, it is necessary to maintain some pressure and air flow offset between the corridor and each room on the corridor in order to assure the desired containment/isolation. However, since air flow conditions in a room, in the corridor, and between the two are not static, but may undergo both small and relatively large changes, a control system is required which can respond to selected conditions which may require a change in offset to thereby maintain desired isolation/ containment. For example, under ordinary conditions when a door is closed between a pressurized room and an adjacent corridor, an air flow velocity between the two of as little as 100 cubic feet per minute (cfm) may be adequate for containment/isolation purposes, and air flow velocities of this magnitude may be utilized, particularly when the air flow volume through the room is relatively low. Such low air flow is desirable since it minimizes energy utilization. However, such an air flow is not considered adequate when the door is open (see ANSI Z9.5 Standard). One reason for this is that there may be an appreciable temperature difference between the pressurized room and the adjoining space resulting in a thermal exchange of warmer air flowing in one direction at the top of the doorway and cooler air flowing in the opposite direction near the floor. An air flow velocity of at least 50 fpm is required to inhibit such thermal exchange under normal conditions and a flow rate of 100 fpm is more desirable to assure isolation/containment. Since for a typical 3'.times.7' open doorway, 1,050 to 2,100 cubic feet per minute (cfm) is therefore required for containment, and this volume is independent of the size of the room or of the cfm of the pressurized room supply and exhaust, the arbitrary 10% "offset" of the room total ventilation rate which is frequently used as the benchmark for the offset is not adequate when the door is open and an increase in air flow offset may be required when this occurs. Similarly, when the door is closed, this offset volume should drop back to the more typical offset volume of 100 to 200 cfm in order to save energy.
Another reason for changing the offset air volume would be when it is desired to change a room from a positive offset to a negative offset or vice versa. This can be desirable in a hospital isolation room where flexible use of the room for either negative isolation or containment, for example for a tuberculosis patient, may be needed one day, and a positive protective isolation is desired on another day for a patient with AIDS or another immunodeficiency disease. Consequently, the offset air volume of the room may need to be changed from a negative 100 cfm to a positive 100 cfm. Similar requirements can exist in animal research facilities or in flexible-use lab facilities of other kinds. A particular problem in this situation is that the corridor typically has a fixed air flow which is based on the projected offsets for each of the rooms serviced by the corridor. Thus, if there are five rooms each having an offset of -100 cfm, the air flow into the corridor might be 500 cfm. However, if one of these rooms is changed so as to be positively pressurized to 100 cfm, the net offset is only 300 cfm but the air flow into the corridor is 500 cfm resulting in an lair flow imbalance.
Further, there may be circumstances where for energy conservation or other reasons, it may be desirable to have a room offset that varies based for example on a percentage of the actual exhaust or supply volume rather than being a fixed percentage of the maximum possible exhaust or supply volume. Such a change may either be continuous or may be staged or stepped, being for example 200 cfm for exhaust volumes between 1,000 and 2,000 cfm of exhaust volume and 100 cfm for volumes of exhaust below 1,000 cfm.
Further, when a substantial change occurs in either the room or the external space/corridor, a change in offset air volume may be required to maintain balance. For example, if there is an emergency situation in a laboratory, for example a spill of toxic material, the fume hood in the laboratory may switch or be switched to a high volume condition resulting in large amounts of air being exhausted from the room. Depending on the supply capacity available to the room, this may cause a corresponding increase in the air flow offset between the room and the adjacent corridor.
However, when a change in air flow offset occurs, effective means is required for controlling the corresponding or counterbalancing offset or transfer from the adjoining space or corridor to prevent large imbalances in the corridor or even in the entire building's pressurization. Left uncompensated, a large variation in the offset air flow for one room could severely affect the pressurization of a corridor which could in turn affect the relative pressure difference and offset volumes between the corridor and other pressurized rooms on the same corridor. In a worst case scenario, this could permit loss of pressure differential in another pressurized room on the corridor which room has a small pressure offset, permitting, for example toxic fumes to enter the room from fume hood therein, and possibly even permitting such fumes or other contaminants to enter the corridor. Negative pressure in the corridor could also make it more difficult to open doors, thus impeding the ability of occupants of the various rooms to escape from the area. This scenario is clearly undesirable.
A related problem is a requirement in some applications that the corridor or other common space be isolated from offset changes required in a given room. This, among other things, improves isolators/ containment between the room and corridor, minimizes potential interaction between rooms on the same corridor and eliminates the need to make balancing changes in the corridor or compensate for desired offset changes for the room. A simple and effective way of achieving this objective does not currently exist.
One prior art system which attempted to deal with this problem involved measuring the differential pressure between the room and the corridor and then controlling the supply of air into the room or the exhaust of air from the room to maintain a set value of room pressure. Such system also used a differential pressure sensor to measure the pressure of the corridor versus some reference point or location either inside or outside the building. A controller accepts the sensed pressure value and then controls a supply valve, damper or equivalent element to provide proper corridor pressure. One problem with this system is that the set point pressure values are very low, resulting in the signals being very noisy and subject to disturbance by walking down the, corridor, wind loads on the building, doors to other areas opening and closing, etc. The result is an inaccurate matching of the offset air volume, slow response time and poor stability of control.
Other systems may, for example, control supply volume into a room and/or exhaust volume from the room based on supply volume from other rooms feeding into the pressurized room but do not directly control the offset air flow between the sealed room and external spaces.
A need therefore exists for an improved control system for use in facilities having one or more pressurized rooms for facilitating selected air flow offsets changes while maintaining a desired air flow balance between the rooms and an external space connected to the rooms.